The present invention is related to a fluorescent X-ray analysis method and a fluorescent X-ray analysis apparatus, particularly to the fluorescent X-ray analysis method which is suitable for detecting, at a high speed, environmental hazardous substances which may be mixed into mechanical parts for electric equipment and electronic equipment which parts have various compositions.
Recently, risk of the environmental hazardous substances which may be contained in parts which constitute electronic or electric equipment has been pointed out, and the contents of the environmental hazardous substances are restricted by laws or regulations in some countries and states. Directive of RoHs (Restriction of the use of certain Hazardous Substances in electrical and electronic equipment) will become effective as of July, 2006. This RoHs Directive inhibits the use of mechanical parts which contains cadmium (Cd), lead (Pb), mercury (Hg), certain brominated flame retardants (Polybrominated Biphenyls (PBB) and Polybrominated diphenyl ether (PBDE)) or hexavalent chromium (Cr(VI)) in an amount above threshold value. The threshold values for Cd, Pb, Hg, PBB, PBDE and Hg are respectively 1000 ppm, while the threshold value for Cr(VI) is 100 ppm. For this reason, it is necessary for electric and electronic equipment manufacturing companies to confirm that each part does not contain the environmental hazardous substance(s) above the threshold value.
As one method for measuring the content of a trace element, a fluorescent X-ray analysis method has been generally employed, which has a sensitivity of several tens ppm and allows nondestructive measurement. The procedure for quantifying the content(s) of element(s) contained in a sample by the fluorescent X-ray analysis is generally well known. For example, Japanese Kokai (Laid-Open) Patent Publication No. 8-43329/1996(A) discloses an example of the procedure.
A conventional procedure of the fluorescent X-ray analysis method is described below with reference to FIG. 3. As shown in FIG. 3, voltage and electric current conditions for an X-ray tube, a quantitative analysis method, and a measuring time (t) are set in the step 301. Next, the measuring is started (see the step 302). Subsequently, the measurement is carried out over the time “t” (see the step 303) and the measurement is finished (see the step 304). After finishing the measurement, the concentration(s) of the element(s) contained in a sample is calculated and the accuracy (standard deviation) of this calculation result is calculated so that the results as to the concentration(s) and the accuracy are obtained. Those results are displayed with displaying means such as an LCD and printed out by a printer (see the step 306).
Two major methods can be used as a quantification method for calculating the concentrations (that is, contents) of the elements contained in the sample. One is a “calibration curve method” wherein a calibration curve has been obtained by previously measuring the content of a target element and a shape of a spectrum, and the shape of the spectrum for the sample is compared with the calibration curve so as to determine the content of the element. The other is a “fundamental parameter (FP) method” wherein all the contained elements are identified and the content of each element is calculated from the spectrum (that is, the all the elements in the sample are quantified (the total of the elements is 100%)) on the assumption that the all the contained elements are reflected on the spectrum.
In general, the contents of trace elements contained in the sample made of, for example, a plastic which contains light elements such as C, H and O in large amounts are quantified by the calibration curve method, since the fluorescent light emission from the light elements is small. On the other hand, the quantification of the trace elements contained in the sample made of middle elements or heavy elements such as iron, zinc and tin is generally carried out by the FP method.
The conventional analysis methods, however, have the following problems:
(1) The exact results as to the concentrations may not be obtained unless the measuring parameters are exactly input, as carried out in the step 301; and
(2) The measuring time tends to be set unnecessarily longer in order to determine the concentration(s) of the elements in the sample more accurately.
The problem (1) is particularly desired to be solved in the situation where a wide variety of parts should be analyzed quickly so as to comply with the RoHs directive. The determination of the measuring time and the selection of the quantification method (the calibration curve method or the FP method) are generally made according to the judgment by an operator. The judgment includes observing the sample to determine whether it is made of a metal or a plastic, based on his/her experience or information which has been given to him/her previously. The skill is required for determining the type or the kind of an unknown sample based on his/her experience, which makes the measurement inefficient. Particularly, the elements have to be quantified by the calibration curve method when the sample is made of a light metal such as aluminum or magnesium. The appearance of such a sample, however, may not be different from that of a sample made of a heavy metal such as iron. For this reason, even the skilled operator may often misjudge. Such misjudgment gives an erroneous result and causes confusion, and requires re-measuring, which reduces the measuring efficiency. The operator may avoid such misjudgment by obtaining the information as to the sample previously. However, the analysis conducted after obtaining the information cannot be said as an “unknown sample analysis” in a precise sense. This means that an effort is required to gathering the information, which reduces the measuring efficiency.
Next, the problem (2) is described by an example. Conventionally, when the content of Cd (which may be referred to as a “Cd concentration”) in a plastic resin is measured by a fluorescent X-ray analysis method, the measuring time “t” is determined at the beginning of the measurement (as in step 301 in FIG. 3). In order to improve the accuracy of the measurement, the measuring time “t” is required to be very long, such as 200 seconds in the light of possibility of the presence of elements other than Cd. However, the volt and electric current in the X-ray tube may be set higher since the sample is a plastic-based one. Therefore, the 200-second measurement per sample may reduce the measurement efficiency in the situation where a large number of parts are required to be analyzed under the RoHs directive. The experiments conducted by the inventors showed that in the case where the Cd concentration such as 20 ppm is measured, 10 seconds is sufficient as the measuring time “t. ” In other words, whether the measuring time “t” is 10 seconds or 200 seconds, the results are the same from the viewpoint that the correct judgment is made as to whether or not the Cd concentration of the plastic sample is below the threshold value stipulated in the RoHs directive. On the other hand, it is necessary to restrict the volt and electric current in the X-ray tube upon measuring the Cd concentration in the sample whose main component is iron since the iron emits a fluorescent X-ray in a large amount. For this reason, a sufficiently long measuring time is required to obtain the counts of fluorescent X-ray from Cd in the sample with a sufficient accuracy. The experiment by the inventors showed that the measuring time should be about 100 seconds when measuring the Cd concentration of about 20 ppm for the sample whose main component is iron.